[geeks] How to take the flicker out of LED Christmas lights?

[Doug McLaren]

On Thu, Nov 23, 2006 at 01:12:53AM -0500, John Francini wrote:

| Since they're cold devices as well as diodes, they run at either 30 
| Hz or 60 Hz (depending on whether there's one or two diodes in each 
| "bulb", wired in parallel, and with opposite polarity).

Actually, it would be 60 Hz or 120 Hz, depending on how things are
wired.  I'm guessing 60 Hz, with each bulb/LED only firing once per
cycle and being off the rest of the cycle.

Actually, if they did it the way you're talking, with two LEDs in each
bulb, each with the opposite polarity, that would help with the
flicker tremendously.  Not only would the frequency double to 120 Hz,
but the (total) bulb wouldn't be completely dark over half the time --
it would only be almost completely dark a small part of each cycle.

(Even incadescent bulbs on 60 Hz AC flicker at 120 Hz, but 1) that's
so fast it's hard to see for the human eye, and 2) the filament
doesn't have time to cool in between cycles, so that evens it out very
effectively.  But if you were to put a single diode in series with
your incadescent bulb, the flicker would drop 60 Hz (I'll ignore
harmonics) and with no power at all half the time, the flicker would
become very noticible.  And the bulb would be dimmer, of course.)

http://en.wikipedia.org/wiki/Image:Rectified_waves.png may help
understand what I'm talking about when I talk about going from 60 to
120 Hz.  Note that the picture includes *3* cycles of the signal, and
remember that power is generally porportional to voltage squared (and
if you square a negative value, you get a positive result.)

| Second, if I recall my electronics correctly, the 120 VAC is RMS, but 
| the peak-to-peak voltage is somewhat higher, isn't it?

Yes, but if you rectify (with a bridge rectifier) and then smooth out
that 120 VAC RMS, you should get approximately 120 VDC -- that's
pretty much what RMS means.  (You'll actually get a little less due to
losses in the rectifier, but the loss should be small.)

Also, since your LEDs will be powered 100% of the time rather than a
little less than half, they'll be considerably brighter.  That might
be OK, or you might want to work around it somehow.

(Simply using a single diode rather than a bridge rectifier will give
you a lower final voltage, but it'll fluctuate a lot more, so you'll
need a lot bigger capacitator or coil to smooth things out.)

| Questions (as I see them)...
| 
| Is this worthwhile?

This is a personal question, only answerable by you.  Is it cost
effective?  No way -- just go out and buy some incadescent X-mas
lights if you want no flicker!  Is it geeky?  Of course!  Is it geeky
enough to be worth doing?  Up to you ...

Is it possible?  I'll bet it is, and I'll bet you're on the right
track.
 
| Can one readily find rectifiers, capacitors, etc. that can carry
| 120V at, say, up to 15 amperes (to be safe) without needing
| industrial-grade components?

15 amps?  Do you *really* have 1600 watts of LED X-mas lights?  (I'll
bet that would be impressive!)

Even one amp should be massively overkill.  For safety, add a fuse!
(And even a one amp fuse is way too high.)

I'm not sure, but I'm guessing that finding rectifiers and
capacitators that can do 120 VDC will not be difficult.

You also don't need a capacitator -- a homemade coil in series with
your circuit should provide the same effect.  Though simply using a
bridge rectifier might be sufficient, with no coil/capactator at all.

If I correctly remember how a switching power supply works, I think
that any old PC power supply should have an appropriate bridge
rectifier in it that can do 120 or 240 VAC.

   http://en.wikipedia.org/wiki/Switched-mode_power_supply

And do be aware that 120 volts is dangerous, so be careful!

--
Doug McLaren, dougmc at frenzied.us        If Murphy's Law can go wrong, it will.